Semiconductor substrate and method of fabricating semiconductor device

ABSTRACT

A semiconductor substrate and a method of fabricating a semiconductor device are provided. An oxide film ( 13 ) is formed by oxidizing an edge section and a lower major surface of an SOI substrate ( 10 ). This oxidizing step is performed in a manner similar to LOCOS (Local Oxide of Silicon) oxidation by using an oxide film ( 11 ) exposed on the edge section and lower major surface of the SOI substrate ( 10 ) as an underlying oxide film. Then, the thickness of the oxide film ( 13 ) is greater than that of the oxide film ( 11 ) on the edge section and lower major surface of the SOI substrate ( 10 ). The semiconductor substrate prevents particles of dust from being produced at the edge thereof.

This application is a division of Ser. No. 09/055,903, filed Apr. 7,1998 now U.S. Pat. No. 6,150,696.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor substrate and a methodof fabricating a semiconductor device and, more particularly, to asemiconductor substrate and a method of fabricating a semiconductordevice which prevent particles of dust from being produced at an edge ofthe substrate.

2. Description of the Background Art

An SOI (silicon on insulator) device including a semiconductor elementformed on an SOI substrate is superior to a bulk device in its decreasedjunction capacitance and improved device isolation breakdown voltage,but has inherent problems to be described below.

FIG. 40 is a sectional view of an SOI substrate 10. The SOI substrate 10has a triple layer structure comprising a silicon substrate 1, a buriedoxide film 2 formed in an upper major surface of the silicon substrate1, and a single crystalline silicon layer (referred to hereinafter as anSOI layer) 3 formed on the buried oxide film 2. A polysilicon layer 4 isformed on edges and a lower major surface of the single crystallinesilicon substrate 1. The polysilicon layer 4 is provided to gettercontaminants such as heavy metal provided during wafer fabrication stepsand a transistor wafer process. A structure having such a polysiliconlayer is known as a poly-back-coat structure (PBC structure).

Methods of fabricating the SOI substrate include a SIMOX (separation byimplanted oxygen) method and a bonding method. The SOI substratefabricated by the SIMOX method (SIMOX substrate) is employed as anexample in the following description.

In the SIMOX method, oxygen ions are implanted into a single crystallinesilicon substrate at a dose of, for example, 0.4×10¹⁸/cm² to 3×10¹⁸/cm²,and thereafter the silicon substrate is annealed at a temperature ofabout 1350° C. to provide the SOI structure.

FIG. 41 is a partial detailed view of an edge of the SOI substrate 10.For purposes of explanation, the semiconductor substrate is divided intofour sections: an upper major surface (on which semiconductor elementsare to be formed), a central section of the upper major surface(including active regions), an edge section including a sectionsurrounding the central section and side surfaces, and a lower majorsurface.

FIG. 41 shows an area X in detail in which the buried oxide film 2 andthe SOI layer 3 meet the polysilicon layer 4. As illustrated in FIG. 41,since the edge section has a curved surface having a great curvature,vertically directed oxygen ions are implanted in a slanting directioninto the edge section, decreasing an effective implantation energy inthe edge section. The result is the reduction in the thickness of theburied oxide film 2 and the SOI layer 3 in the edge section, creating astructure wherein the SOI layer 3 is prone to exfoliate.

Additionally, the step of thinning the SOI layer 3 during thefabrication of the SOI device promotes the exfoliation of the SOI layer3. The step of thinning the SOI layer 3 is described with reference toFIGS. 42 and 43.

The SOI layer 3 in the SOI substrate 10 has a suitable thickness asshown in FIG. 42 when the substrate is fabricated. The step of thinningthe SOI layer 3 is to suitably reduce the thickness of the SOI layer 3in accordance with the specs of a desired semiconductor device, andcomprises oxidizing the SO1 layer 3 and removing the resultant oxidefilm to adjust the thickness of the SOI layer 3.

FIG. 43 shows an oxide film 5 formed on the SOI layer 3. The thicknessof the oxide film 5 is generally determined based on the thickness ofthe SOI layer 3 in the central section of the SOI substrate 10, that is,semiconductor element formation regions (active regions). The problemsthat arise herein are the reduced thickness of the SOI layer 3 in theedge section of the SOI substrate 10 as above described, and theformation of the polysilicon layer 4 in the edge section of the SOIsubstrate 10. An area Y shown in FIG. 42 is illustrated in more detailin FIG. 44, and an area Z shown in FIG. 43 is illustrated in more detailin FIG. 45. FIG. 46 shows the edge section after the removal of theoxide film 5.

As illustrated in FIG. 44, the polysilicon layer 4 is comprised of amultiplicity of single crystal grains GP. Because of individuallydifferent crystal orientations of the single crystal grains GP, theoxygen ions are implanted to different depths due to channeling, causingthe buried oxide film 2 to be formed at varied depths.

Further, different oxidation rates of the polysilicon layer 4 dependingon the crystal orientations of the single crystal grains GP result indifferent thicknesses of the oxide film 5 in accordance with therespective single crystal grains GP as shown in FIG. 45 after theoxidation of the polysilicon layer 4.

The reduced thickness of the SOI layer 3 in the edge section of the SOIsubstrate 10 might cause the oxide film 5 to be contact with the buriedoxide film 2 depending on the single crystal grains GP and cause the SOIlayer 3 to be completely oxidized. In such cases, part of the SOI layer3 might be surrounded by the buried oxide film 2 and the oxide film 5.For example, an SOI layer 30 shown in FIG. 45 is surrounded by the oxidefilm 5 and the buried oxide film 2.

When wet etching is performed on the oxide film 5 using an etchant suchas hydrofluoric acid for thinning the SOI layer 3 in the SOI substrate10 under the above described conditions, the buried oxide film 2 as wellas the oxide film 5 is etched as shown in FIG. 46. Then, the SOI layer30 is lifted off into a particle suspended in the etchant. In somecases, the SOI layer 30 might adhere to the central section of the SOIsubstrate 10. The adhesion of particles to the semiconductor elementformation regions causes the formation failures of semiconductorelements and, accordingly, the decrease in fabrication yield.

As above described, the background art semiconductor substrate,particularly the SOI substrate, has the drawback that the SOI layer inthe edge section of the substrate exfoliates into particles to cause thedecrease in fabrication yield. The production of the particles is also aproblem for semiconductor substrates other than the SOI substrate.

SUMMARY OF THE INVENTION

A first aspect of the present invention is intended for a method offabricating a semiconductor device using a semiconductor substratehaving a first major surface, a second major surface opposite from thefirst major surface, and a side surface, the first major surfaceincluding a central section in which active regions are to be formed anda peripheral section, the peripheral section and the side surfacedefining an edge section. According to the present invention, the methodcomprises the steps of: (a) forming a first oxide film so as to coverthe central section and the edge section of the semiconductor substrate;(b) selectively forming an oxidation-resistant film on the first oxidefilm in the central section; (c) further oxidizing the edge section ofthe semiconductor substrate using the oxidation-resistant film as a maskto form a second oxide film in the edge section, the second oxide filmbeing thicker than the first oxide film; and (d) forming semiconductorelements in the active regions.

Preferably, according to a second aspect of the present invention, inthe method of the first aspect, the semiconductor substrate is an SOIsubstrate formed by a SIMOX technique; the semiconductor substratecomprises a buried oxide film and an SOI layer formed in a sequentiallystacked relation in the entire first major surface; and the step (c)comprises the step of (c-1) forming the second oxide film so as tocompletely oxidize the SOI layer extending in the edge section and tooxidize part of the edge section which has not been oxidized.

Preferably, according to a third aspect of the present invention, in themethod of the first aspect, the semiconductor substrate is an SOIsubstrate formed by a bonding technique; the semiconductor substratecomprises an on-substrate oxide film and an SOI layer formed in asequentially stacked relation on the entire first major surface; and thestep (c) comprises the step of (c-1) forming the second oxide film so asto completely oxidize the SOI layer extending in the edge section and tooxidize part of the edge section which has not been oxidized.

Preferably, according to a fourth aspect of the present invention, inthe method of the first aspect, the semiconductor substrate is a bulksilicon substrate; the semiconductor substrate comprises a polysiliconlayer formed on the edge section and the second major surface; and thestep (c) comprises the step of (c-1) forming the second oxide film sothat the polysilicon layer is not completely oxidized.

Preferably, according to a fifth aspect of the present invention, in themethod of the second aspect, the step (a) comprises the step of formingthe first oxide film so that the thickness of the SOI layer in thecentral section is reduced to a thickness conforming to formation ofsemiconductor elements.

Preferably, according to a sixth aspect of the present invention, in themethod of the fifth aspect, the step (b) comprises the step of forming apattern of the oxidation-resistant film in accordance with the patternof a field oxide film defining the active regions in the centralsection; and the step (c) comprises the step of forming the second oxidefilm as the field oxide film in accordance with the pattern of theoxidation-resistant film in the central section.

A seventh aspect of the present invention is intended for a method offabricating a semiconductor device using a semiconductor substratehaving a first major surface, a second major surface opposite from thefirst major surface, and a side surface, the first major surfaceincluding a central section in which active regions are to be formed anda peripheral section, the peripheral section and the side surfacedefining an edge section. According to the present invention, the methodcomprises the steps of: (a) forming an oxide film so as to cover thecentral section and the edge section of the semiconductor substrate; (b)forming a resist mask on the oxide film except in the central section;(c) selectively removing the oxide film in the central section using theresist mask as an etching mask to expose the semiconductor substrate,with the oxide film left in the edge section; and (d) formingsemiconductor elements in the active regions.

Preferably, according to an eighth aspect of the present invention, themethod of the seventh aspect further comprises the step of (e) formingan oxidation-resistant film on the oxide film in the edge section.

Preferably, according to a ninth aspect of the present invention, in themethod of the seventh aspect, the semiconductor substrate is an SOIsubstrate formed by a SIMOX technique; the semiconductor substratecomprises a buried oxide film and an SOI layer formed in a sequentiallystacked relation in the entire first major surface; and the step (a)comprises the step of forming the oxide film so that the thickness ofthe SOI layer in the central section is reduced to a thicknessconforming to formation of semiconductor elements.

A tenth aspect of the present invention is intended for a method offabricating a semiconductor device using a semiconductor substratehaving a first major surface, a second major surface opposite from thefirst major surface, and a side surface, the first major surfaceincluding a central section in which active regions are to be formed anda peripheral section, the peripheral section and the side surfacedefining an edge section, the semiconductor substrate being an SOIsubstrate formed by a SIMOX technique, the semiconductor substrateincluding a buried oxide film and an SOI layer formed in a sequentiallystacked relation in the entire first major surface. According to thepresent invention, the method comprises the steps of: (a) forming afirst oxide film so as to cover the central section and the edge sectionof the semiconductor substrate; (b) selectively forming a resist mask onthe first oxide film in the central section; (c) selectively removingthe first oxide film and the SOI layer in the edge section of thesemiconductor substrate using the resist mask as an etching mask toexpose the buried oxide film; (d) further oxidizing the first oxide filmunder the resist mask to form a second oxide film thicker than the firstoxide film and to increase the thickness of the buried oxide filmexposed; and (e) forming semiconductor elements in the active regions.

An eleventh aspect of the present invention is intended for a method offabricating a semiconductor device using a semiconductor substratehaving a first major surface, a second major surface opposite from thefirst major surface, and a side surface, the first major surfaceincluding a central section in which active regions are to be formed anda peripheral section, the peripheral section and the side surfacedefining an edge section, the semiconductor substrate being an SOIsubstrate formed by a SIMOX technique, the semiconductor substrateincluding a buried oxide film and an SOI layer formed in a sequentiallystacked relation in the entire first major surface. According to thepresent invention, the method comprises the steps of: (a) forming afirst oxide film so as to cover the central section and the edge sectionof the semiconductor substrate; (b) selectively forming a resist mask onthe first oxide film in the central section; (c) selectively removingthe first oxide film, the SOI layer and the buried oxide film in theedge section of the semiconductor substrate by dry etching using theresist mask as an etching mask to expose an underlying substrate underthe SOI layer; (d) further oxidizing the first oxide film under theresist mask to form a second oxide film thicker than the first oxidefilm and to form a third oxide film on the underlying substrate exposed;and (e) forming semiconductor elements in the active regions.

Preferably, according to a twelfth aspect of the present invention, inthe method of the tenth aspect, the step (d) comprises the step offorming the second oxide film so that the thickness of the SOI layer inthe central section is reduced to a thickness conforming to formation ofsemiconductor elements.

A thirteenth aspect of the present invention is intended for asemiconductor substrate having a first major surface, a second majorsurface opposite from the first major surface, and a side surface, thefirst major surface including a central section in which active regionsare to be formed and a peripheral section, the peripheral section andthe side surface defining an edge section. According to the presentinvention, the semiconductor substrate comprises: a buried oxide filmand an SOI layer formed in a sequentially stacked relation in the entirefirst major surface; and an oxide film formed in the edge section andhaving a thickness reaching the buried oxide film.

A fourteenth aspect of the present invention is intended for asemiconductor substrate having a first major surface, a second majorsurface opposite from the first major surface, and a side surface, thefirst major surface including a central section in which active regionsare to be formed and a peripheral section, the peripheral section andthe side surface defining an edge section. According to the presentinvention, the semiconductor substrate comprises: a buried oxide filmand an SOI layer formed in a sequentially stacked relation in the firstmajor surface, wherein the buried oxide film contains silicon islands,and wherein the density of the silicon islands is lower in the buriedoxide film extending in the edge section than in the buried oxide filmin the central section.

A fifteenth aspect of the present invention is intended for asemiconductor substrate having a first major surface, a second majorsurface opposite from the first major surface, and a side surface, thefirst major surface including a central section in which active regionsare to be formed and a peripheral section, the peripheral section andthe side surface defining an edge section. According to the presentinvention, the semiconductor substrate comprises: a buried oxide filmand an SOI layer formed in a sequentially stacked relation in the firstmajor surface, wherein the buried oxide film contains silicon islands,and wherein the buried oxide film and the SOI layer are not formed inthe edge section.

In accordance with the method of the first aspect of the presentinvention, the relatively thick second oxide film is formed on the edgesection. If a layer which is prone to exfoliate due to wet etching ispresent on the edge section and second major surface, the second oxidefilm functions as a protective film to eliminate the problem that partof the layer which is prone to exfoliate is lifted off into particlessuspending in the etchant. This prevents the formation failures of thesemiconductor elements resulting from the presence of the particles,increasing the fabrication yield.

In accordance with the method of the second aspect of the presentinvention, the second oxide film is formed on the edge section of theSOI substrate formed by the SIMOX technique so that the SOI layer in theedge section is completely oxidized and the part of the edge sectionwhich has not been oxidized is oxidized. Thus, the SOI layer which isprone to exfoliate due to wet etching is protected, and eliminated isthe problem that part of the SOI layer is lifted off into particlessuspending in the etchant. This prevents the formation failures of thesemiconductor elements resulting from the presence of the particles,increasing the fabrication yield.

In accordance with the method of the third aspect of the presentinvention, the second oxide film is formed on the edge section of theSOI substrate formed by the bonding technique so that the SOI layer inthe edge section is completely oxidized and the part of the edge sectionwhich has not been oxidized is oxidized. If the edge section of theon-substrate oxide film and SOI layer is not perfectly chamfered toprovide a continuously uneven or rough peripheral configuration in planview, the uneven or rough periphery is prevented from exfoliating intoparticles, and the edge section of the on-substrate oxide film isprevented from being partially removed during wet etching.

In accordance with the method of the fourth aspect of the presentinvention, the bulk silicon substrate includes the polysilicon layerformed on the edge section and second major surface. In this case, thepresence of the second oxide film formed on the edge section so that thepolysilicon layer is not completely oxidized prevents the polysiliconlayer from exfoliating during wet etching due to a structure inherent inthe polysilicon layer.

In accordance with the method of the fifth aspect of the presentinvention, the thickness of the first oxide film is made suitable forthinning the SOI layer. This eliminates the need to reduce the thicknessof the SOI layer in a subsequent step, simplifying the steps ofprocessing the semiconductor substrate.

The method of the sixth aspect of the present invention is capable offorming the second oxide film and the field oxide film at the same time,simplifying the steps of processing the semiconductor substrate.

The method of the seventh aspect of the present invention is capable ofreadily and conveniently forming the oxide film on the edge section ofthe semiconductor substrate to significantly simplify the steps ofprocessing the semiconductor substrate, reducing processing costs.

In accordance with the method of the eighth aspect of the presentinvention, the edge section of the semiconductor substrate is rigidlyprotected by the oxide film and the first oxidation-resistant film.

In accordance with the method of the ninth aspect of the presentinvention, the thickness of the oxide film is made suitable for thinningthe SOI layer. This eliminates the need to reduce the thickness of theSOI layer in a subsequent step, simplifying the steps of processing thesemiconductor substrate.

In accordance with the method of the tenth aspect of the presentinvention, the buried oxide film exposed in the edge section of the SOIsubstrate is exposed to oxygen serving as an oxidizing agent in the step(d). Thus, as the oxygen is diffused in the buried oxide film to reachsilicon islands inherent in the buried oxide film of the SOI substrateformed by the SIMOX technique, the oxygen reacts with silicon to form asilicon oxide film, resulting in disappearance of the silicon islands.The result is the reduced number of silicon islands in the buried oxidefilm in the edge section of the SOI substrate. If the buried oxide filmis removed by wet etching, the silicon islands are prevented from beinglifted off into particles.

In accordance with the method of the eleventh aspect of the presentinvention, dry etching is used to selectively remove the first oxidefilm, the SOI layer, and the buried oxide film in the edge section ofthe semiconductor substrate. This permits the silicon islands inherentin the buried oxide film of the SOI substrate formed by the SIMOXtechnique to disappear in the edge section of the semiconductorsubstrate, preventing the silicon islands from being lifted off intoparticles during wet etching.

In accordance with the method of the twelfth aspect of the presentinvention, the thickness of the second oxide film is made suitable forthinning the SOI layer. This eliminates the need to reduce the thicknessof the SOI layer in a subsequent step, simplifying the steps ofprocessing the semiconductor substrate.

In accordance with the semiconductor substrate of the thirteenth aspectof the present invention, the presence of the oxide film formed on theedge section of the semiconductor substrate and having the thicknessreaching the buried oxide film may protect the SOI layer which is proneto exfoliate due to wet etching to eliminate the problem that part ofthe SOI layer is lifted off into particles suspending in the etchant.This prevents the formation failures of the semiconductor elementsresulting from the presence of the particles, providing semiconductorsubstrates with an increased fabrication yield.

In accordance with the semiconductor substrate of the fourteenth aspectof the present invention, the density of the silicon islands is lower inthe buried oxide film in the edge section of the semiconductor substratethan in the buried oxide film in the central section of the first majorsurface. If the buried oxide film is removed by wet etching, thesemiconductor substrate which prevents the silicon islands from beinglifted off into particles is accomplished.

In accordance with the semiconductor substrate of the fifteenth aspectof the present invention, the buried oxide film and the SOI layer arenot formed in the edge section of the semiconductor substrate. Thisaccomplishes the semiconductor substrate which prevents the siliconislands from being lifted off into particles during wet etching.

It is therefore an object of the present invention to provide asemiconductor substrate and a method of fabricating a semiconductordevice which prevent particles of dust from being produced at an edge ofthe substrate.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view illustrating the step of processing asemiconductor substrate according to a first preferred embodiment of thepresent invention;

FIG. 2 is a plan view illustrating the step of processing thesemiconductor substrate according to the first preferred embodiment ofthe present invention;

FIGS. 3 through 6 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to the first preferredembodiment of the present invention;

FIG. 7 is a sectional view showing an endmost edge of the semiconductorsubstrate;

FIGS. 8 through 11 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to a second preferredembodiment of the present invention;

FIGS. 12 through 14 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to a modification ofthe second preferred embodiment of the present invention;

FIGS. 15 through 18 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to a third preferredembodiment of the present invention;

FIGS. 19 and 20 are sectional views illustrating the steps of processingthe semiconductor substrate according to a fourth preferred embodimentof the present invention;

FIGS. 21 through 23 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to a fifth preferredembodiment of the present invention;

FIGS. 24 through 28 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to a sixth preferredembodiment of the present invention;

FIGS. 29 through 31 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to a seventh preferredembodiment of the present invention;

FIG. 32 is a plan view illustrating the steps of processing thesemiconductor substrate according to the seventh preferred embodiment ofthe present invention;

FIGS. 33 through 39 are sectional views illustrating the steps ofprocessing the semiconductor substrate according to the seventhpreferred embodiment of the present invention;

FIG. 40 is a sectional view of an SOI substrate;

FIGS. 41 through 43 are sectional views illustrating the conventionalsteps of processing the SOI substrate; and

FIGS. 44 through 46 are sectional views illustrating problems in theconventional steps of processing the SOI substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<A. First Preferred Embodiment>

A semiconductor substrate and a method of fabricating a semiconductordevice according to a first preferred embodiment of the presentinvention will now be described with reference to FIGS. 1 through 6. Forpurposes of explanation, the semiconductor substrate is divided intofour sections: an upper major surface (on which semiconductor elementsare to be formed), a central section of the upper major surface(including active regions), an edge section including a sectionsurrounding the central section and side surfaces, and a lower majorsurface.

<A-1. Processing Method>

Referring to FIG. 1, a silicon oxide film (referred to hereinafter as anoxide film) 11 having a thickness of 100 to 400 angstroms is formed soas to entirely cover an SOI substrate 10. The oxide film 11 (first oxidefilm) may be formed by thermally oxidizing the SOI substrate 10 at atemperature on the order of 700 to 1100° C. or by the CVD process at atemperature on the order of 600 to 850° C.

The SOI substrate 10 has a triple layer structure comprising a singlecrystalline silicon substrate (bulk silicon substrate) 1, a buried oxidefilm 2 formed on the upper major surface of the single crystallinesilicon substrate 1, and a single crystalline silicon layer (referred tohereinafter as an SOI layer) 3 formed on the buried oxide film 2. Apolysilicon layer 4 is formed on the edge section and lower majorsurface of the single crystalline silicon substrate 1. The polysiliconlayer 4 is provided to getter contaminants such as heavy metals providedduring wafer fabrication steps. A structure having such a polysiliconlayer is known as a poly-back-coat structure (PBC structure).

The SOI substrate fabricated by the SIMOX method (SIMOX substrate) isemployed as an example in the following description.

First, a silicon nitride film (referred to hereinafter as a nitridefilm) 12 serving as an oxidation-resistant film and having a thicknessof 1000 to 4000 angstroms is formed by the CVD process at a temperatureon the order of 600 to 850° C. so as to entirely cover the oxide film11.

A resist mask R1 is selectively formed so as to cover the centralsection of the upper major surface (on which the active regions are tobe formed) of the SOI substrate 10. FIG. 2 is a plan view of the SOIsubstrate 10 as viewed from above the upper major surface thereof. Theresist mask R1 is not formed in the edge section of the SOI substrate 10as shown in FIG. 2. The range of the resist mask R1 to be formed is setto entirely cover the active regions wherein semiconductor elements areto be formed.

Dry etching is performed using the resist mask R1 as an etching mask toselectively remove the nitride film 12 so that the nitride film 12 isleft only under the resist mask R1 as shown in FIG. 3. That is, thenitride film 12 is removed and the oxide film 11 is exposed in the edgesection of the SOI substrate 10 which is not covered with the resistmask R1. The nitride film 12 is removed throughout the lower majorsurface of the SOI substrate 10. Wet etching using, for example, thermalphosphoric acid may be employed for removal of the nitride film 12.

In the step shown in FIG. 4, an oxide film 13 (second oxide film) isformed by oxidizing the edge section and the lower major surface of theSOI substrate 10. This oxidizing step is performed in a manner similarto LOCOS (Local Oxide of Silicon) oxidation by using the oxide film 11exposed on the edge section and lower major surface of the SOI substrate10 as an underlying oxide film. The conditions of this oxidizing step tobe selected are such that all of the SOI layer 3 except under thenitride film 12 is oxidized. For example, when the SOI layer 3 under thenitride film 12 has a thickness of 2000 angstroms, the oxide film 13should have a thickness of not less than 5000 angstroms.

Next, in the step shown in FIG. 5, the nitride film 12 is removed, andthen the thickness of the SOI layer 3 which has been positioned underthe nitride film 12 is suitably reduced in accordance with the specs ofa desired semiconductor device. That is, the SOI layer 3 is thinned byfurther oxidizing the oxide film 11 to increase the thickness of theoxide film 11. In this step, the thickness of the oxide film 13 formedon the edge section and lower major surface of the SOI substrate 10 isalso increased. For reduction in the thickness of the SOI layer 3 by1000 angstroms, the oxidizing conditions should be set so that thethickness of the oxide film 11 is increased by 2000 angstroms.

In the step shown in FIG. 6, the thickened oxide film 11 is removed bywet etching.

<A-2. Characteristic Function and Effect>

In the step of thinning the SOI layer 3, as above described, thethickness of the oxide film 13 formed on the edge section and lowermajor surface of the SOI substrate 10 is also reduced. However, sincethe thickness of the oxide film 13 is originally greater than that ofthe oxide film 11 and is increased in the step of thinning the SOI layer3, the oxide film 13 is not completely removed during the etching of theoxide film 11. Further, the oxide film 13 is formed so that the SOIlayer 3 is not left in the edge section and lower major surface of theSOI substrate 10. This eliminates the drawback that part of the SOIlayer 3 is lifted off into particles suspending in the etchant toprevent the formation failures of the semiconductor elements resultingfrom the presence of the particles, increasing the fabrication yield.

In subsequent steps, MOS transistors and bipolar transistors are madeinto the SOI layer 3 on the upper major surface of the SOI substrate 10with the oxide film 13 formed on the edge section and lower majorsurface thereof to construct a DRAM, an SRAM and a logic circuit.

<A-3. Modifications>

Although the present invention is applied to the SOI substrate havingthe PBC structure in the above description, the present invention may beapplied to an SOI substrate having no PBC structure wherein particlesmight be produced.

FIG. 7 is a sectional view of an endmost edge of an SOI substrate 100having no PBC structure. As shown in FIG. 7, when oxide ions areimplanted into the silicon substrate 1 to form the buried oxide film 2,the oxide ions might be implanted into a section which should be the SOIlayer at an endmost edge ME of the SOI substrate 100 to form an oxidefilm therein. Then, the endmost edge ME is not entirely oxidized, butgranular single crystalline silicon regions (referred to hereinafter assilicon islands SI) are partially left and might flow as particles intothe etchant during the wet etching of the oxide film. As described inthe first preferred embodiment, however, the oxide film 13 formed on theedge section and lower major surface prevents the silicon islands SIfrom becoming the particles.

<B. Second Preferred Embodiment>

The first preferred embodiment of the present invention describes thestructure for preventing the SOI layer 3 at the edge of the SOIsubstrate 10 from being partially lifted off into particles. In somecases, however, the silicon islands contained in the buried oxide film 2might be a source of particles.

The silicon islands are discussed below. A plurality of silicon islandsSI are present in the buried oxide film 2 as shown in FIG. 8. Thesilicon islands SI are produced when the buried oxide film 2 is formedby ion implantation, and are inherent in the SIMOX substrate.Specifically, while oxygen ions are implanted into the silicon substrateto form the buried oxide film 2, silicon atoms which do not react withthe oxygen ions are combined to each other during the SIMOX annealingstep to form silicon masses which result in the silicon islands SI.

Since many of the silicon islands SI are present in a relatively deeppart of the buried oxide film 2, the silicon islands SI, in general, donot appear on the surface of the buried oxide film 2 if the buried oxidefilm 2 is etched in some amounts. However, in the edge section of theSOI substrate 10 where the SOI layer 3 and the buried oxide film 2 arerelatively thin as shown in FIG. 8, the silicon islands SI are exposedby etching and, in some cases, lifted off into particles.

A structure for reducing the particles resulting from the siliconislands is described hereinafter in a second preferred embodiment of thesemiconductor substrate and the method of fabricating the semiconductordevice according to the present invention with reference to FIGS. 8through 11.

<B-1. Processing Method>

First, an oxide film 21 (first oxide film) having a thickness of 100 to400 angstroms is formed so as to entirely cover the SOI substrate 10 asshown in FIG. 8. The oxide film 21 may be formed by thermally oxidizingthe SOI substrate 10 at a temperature on the order of 700 to 1100° C. orby the CVD process at a temperature on the order of 600 to 850° C.

Then, a resist mask R2 is selectively formed so as to cover the centralsection of the upper major surface (on which the active regions are tobe formed) of the SOI substrate 10. The range of the resist mask R1 tobe formed is set to entirely cover a region in which the active regionsare to be formed.

Dry etching is performed using the resist mask R2 as an etching mask toselectively remove parts of the oxide film 21 and SOI layer 3 which arenot covered with the resist mask R2 so that the oxide film 21 and theSOI layer 3 are left only under the resist mask R2 as shown in FIG. 9.That is, the oxide film 21 and the SOI layer 3 are removed and theburied oxide film 2 is exposed in the edge section and lower majorsurface of the SO substrate 10 which are not covered with the resistmask R2.

Next, the resist mask R2 is removed, and then the thickness of the SOIlayer 3 which has been positioned under the resist mask R2 is suitablyreduced in accordance with the specs of a desired semiconductor deviceas shown in FIG. 10. That is, the SOI layer 3 is thinned by furtheroxidizing the oxide film 21 to form an oxide film 23 (second oxidefilm). For reduction in the thickness of the SOI layer 3 by 1000angstroms, the oxidizing conditions should be set so that the thicknessof the oxide film 23 is thicker than the oxide film 21 by 2000angstroms. At this time, the buried oxide film 2 in the edge section andlower major surface of the SOI substrate 10 is exposed to oxygen servingas an oxidizing agent. When the oxygen is diffused in the buried oxidefilm 2 to reach the silicon islands SI, the oxygen reacts with siliconto form a silicon oxide film, resulting in the disappearance of thesilicon islands SI.

<B-2. Characteristic Function and Effect>

In the step of thinning the SOI layer 3, as above described, the siliconislands SI are reduced in the edge section of the SOI substrate 10 ifthe oxide film 23 formed for thinning the SOI layer 3 is removed by wetetching as shown in FIG. 11. This prevents the silicon islands SI frombeing lifted off into particles.

Although described above is the disappearance of the silicon islands SIin conjunction with the further oxidization and thickening of the oxidefilm 21 for thinning the SOI layer 3, the silicon islands SI may vanishin other oxidizing steps which do not follow the step wherein the edgesection of the SOI substrate 10 is subjected to wet etching.

<B-3. Modifications of Processing Method>

Although the silicon islands SI vanish in conjunction with the furtheroxidization and thickening of the oxide film 21 for thinning the SOIlayer 3 in the second preferred embodiment of the present invention asabove described, the buried oxide film 2 in the edge section of the SOIsubstrate 10 may be removed to prevent the silicon islands SI frombecoming the particles.

Specifically, as shown in FIG. 12, the oxide film 21 is formed so as toentirely cover the SOI substrate 10, and the resist mask R2 isselectively formed so as to cover the central section of the upper majorsurface of the SOI substrate 10.

Dry etching is performed using the resist mask R2 as an etching mask toselectively remove parts of the oxide film 21, the SOI layer 3 and theburied oxide film 2 which are not covered with the resist mask R2 insequential order so that the oxide film 21, the SOI layer 3 and theburied oxide film 2 are left only under the resist mask R2 as shown inFIG. 13. Such dry etching is also performed on the lower major surfaceof the SOI substrate 10, thereby exposing the silicon substrate 1(underlying substrate) in the edge section and lower major surface ofthe SOI substrate 10 which are not covered with the resist mask R2.

Next, the resist mask R2 is removed, and then the thickness of the SOIlayer 3 which has been positioned under the resist mask R2 is suitablyreduced in accordance with the specs of a desired semiconductor deviceas shown in FIG. 14. That is, the SOI layer 3 is thinned by furtheroxidizing the oxide film 21 to form the oxide film 23. At this time, anoxide film 24 is formed on the exposed surface of the silicon substrate1.

The removal of the buried oxide film 2 in the edge section of the SOIsubstrate 10 by dry etching permits the silicon islands SI to disappearin the edge section of the SOI substrate 10, preventing the siliconislands SI from becoming the particles during the removal by the wetetching of the oxide film 23 formed for thinning the SOI layer 3.

<C. Third Preferred Embodiment>

The first and second preferred embodiments of the present inventiondescribe the structure for preventing the SOI layer 3 in the edgesection of the SIMOX substrate from producing the particles and thestructure for preventing the silicon islands in the buried oxide film ofthe SIMOX substrate from producing the particles. In some cases,however, particles might be produced in the SOI substrate fabricated bythe bonding method (bonded substrate).

The bonded substrate is fabricated by forming an oxide film on an uppermajor surface (on which semiconductor elements are to be formed) of afirst silicon substrate, bonding a second silicon substrate to an uppersurface of the oxide film, and polishing the second silicon substrate toa predetermined thickness, thereby providing an SOI structure. FIG. 15is a sectional view of the edge section of an SOI substrate 200 formedin this manner.

Referring to FIG. 15, the silicon substrate 1, an on-substrate oxidefilm 7 formed on the upper major surface of the silicon substrate 1, anda silicon layer 8 formed on the oxide film 7 constitute an SOIstructure. The on-substrate oxide film 7 corresponds to the buried oxidefilm, and the silicon layer 8 corresponds to the SOI layer.

In the SOI substrate 200 having the above structure, the on-substrateoxide film 7 and the silicon layer 8 are not perfectly chamfered in theedge section, but provide a continuously uneven or rough peripheralconfiguration in plan view in some cases. The uneven or rough peripherysometimes exfoliates into particles during the transportation of thesubstrate.

Further, an etchant might enter the edge section where the on-substrateoxide film 7 is exposed during wet etching to partially remove theon-substrate oxide film 7. FIG. 16 is a detailed view of an area W shownin FIG. 15.

With reference to FIG. 16, part of the on-substrate oxide film 7 at theedge of the SOI layer 8 is removed to cause the SOI layer 8 to partiallydangle. Under these conditions, the SOI layer 8 is liable to exfoliateinto particles.

A structure for reducing the particles in the bonded substrate isdescribed hereinafter in a third preferred embodiment of thesemiconductor substrate and the method of fabricating the semiconductordevice according to the present invention with reference to FIGS. 17 and18.

<C-1. Processing Method>

Referring to FIG. 17, an oxide film 31 (first oxide film) having athickness of 100 to 400 angstroms is formed so as to entirely cover theSOI substrate 200. The oxide film 31 may be formed by thermallyoxidizing the SOI substrate 200 at a temperature on the order of 700 to1100° C. or by the CVD process at a temperature on the order of 600 to850° C.

Then, a nitride film 32 (oxidation-resistant film) having a thickness of1000 to 4000 angstroms is formed by the CVD process at a temperature onthe order of 600 to 850° C. so as to entirely cover the oxide film 31.

Then, a resist mask R3 is selectively formed so as to cover the centralsection of the upper major surface (on which the active regions are tobe formed) of the SOI substrate 200.

Dry etching is performed using the resist mask R3 as an etching mask toselectively remove the nitride film 32, and thereafter the resist maskR3 is removed to leave the nitride film 32 only in the central sectionof the upper major surface. That is, the nitride film 32 is removed andthe oxide film 31 is exposed in an area extending from the edge sectionof the SOI layer 8 to the edge section and lower major surface of thesilicon substrate 1 which has not been covered with the resist ask R3.The nitride film 32 is removed throughout the lower major surface of theSOI substrate 200. Wet etching using, for example, thermal phosphoricacid may be employed for removal of the nitride film 32.

In the step shown in FIG. 18, an oxide film 33 (second oxide film) isformed by oxidizing the area extending from the edge section of the SOIlayer 8 to the edge section and lower major surface of the siliconsubstrate 1. This oxidizing step is performed in a manner similar to theLOCOS oxidation by using the oxide film 31 exposed in the area extendingfrom the edge section of the SOI layer 8 to the edge section and lowermajor surface of the silicon substrate 1 as an underlying oxide film.The conditions of this oxidizing step to be selected are such that allof the SOI layer 8 except under the nitride film 32 is oxidized.

The nitride film 32 is removed, and then the thickness of the SOI layer8 which has been positioned under the nitride film 32 is suitablyreduced in accordance with the specs of a desired semiconductor device.Such a film thinning step of the third preferred embodiment is similarto that of the first preferred embodiment described with reference toFIGS. 5 and 6, and the description thereof will be dispensed withherein.

<C-2. Characteristic Function and Effect>

In the SOI substrate 200 which is the bonded substrate, as abovedescribed, the oxide film 33 is formed in the area extending from theedge section of the SOI layer 8 to the edge section and lower majorsurface of the silicon substrate 1. This prevents the on-substrate oxidefilm 7 and the silicon layer 8 from exfoliating at their edges intoparticles during the transportation of the SOI substrate 200 and due towet etching during the step of thinning the SOI layer 8.

<D. Fourth Preferred Embodiment>

The first to third preferred embodiments of the present inventiondescribe the reduction in the particles in the SOI substrate. In somecases, however, a polysilicon layer might exfoliate into particles in abulk silicon substrate having the PBC structure. Specifically, as statedwith reference to FIG. 44, the polysilicon layer is comprised of amultiplicity of single crystal grains. During the oxidation of thepolysilicon layer, oxygen serving as an oxidizing agent might enter thegaps between the single crystal grains to form oxide films surroundingthe single crystal grains. Under such circumstances, if the oxide filmis removed by wet etching, there is a strong likelihood that the singlecrystal grains are lifted off into particles.

A structure for reducing the particles in the bulk silicon substrate isdescribed hereinafter in a fourth preferred embodiment of thesemiconductor substrate and the method of fabricating the semiconductordevice according to the present invention with reference to FIGS. 19 and20.

<D-1. Processing Method>

With reference to FIG. 19, the polysilicon layer 4 is formed on the edgesection and lower major surface of the single crystalline siliconsubstrate (bulk silicon substrate) 1. A substrate comprised of thesilicon substrate 1 and the polysilicon layer 4 is referred tohereinafter as a silicon substrate 300.

As illustrated in FIG. 19, an oxide film 41 (first oxide film) having athickness of 100 to 400 angstroms is formed so as to entirely cover thesilicon substrate 300. The oxide film 41 may be formed by thermallyoxidizing the silicon substrate 300 at a temperature on the order of 700to 1100° C. or by the CVD process at a temperature on the order of 600to 850° C.

Then, a nitride film 42 (oxidation-resistant film) having a thickness of1000 to 4000 angstroms is formed by the CVD process at a temperature onthe order of 600 to 850° C. so as to entirely cover the oxide film 41.

Then, a resist mask R4 is selectively formed so as to cover the centralsection of the upper major surface (on which the active regions are tobe formed) of the silicon substrate 300.

Dry etching is performed using the resist mask R4 as an etching mask toselectively remove the nitride film 42, and thereafter the resist maskR4 is removed to leave the nitride film 42 only in the central sectionof the upper major surface. That is, the nitride film 42 is removed andthe oxide film 41 is exposed in the edge section of the siliconsubstrate 300 which has not been covered with the resist mask R4. Thenitride film 42 is removed throughout the lower major surface of thesilicon substrate 300. Wet etching using, for example, phosphoric acidat elevated temperatures may be employed for removal of the nitride film42.

In the step shown in FIG. 20, an oxide film 43 (second oxide film) isformed on the edge section and lower major surface of the siliconsubstrate 300. This oxidizing step is performed in a manner similar tothe LOCOS oxidation by using the oxide film 41 exposed in an areaextending from the edge section to the lower major surface of thesilicon substrate 300 as an underlying oxide film. The conditions ofthis oxidizing step to be selected are such that the oxide film 43 isthick enough to preclude all of the polysilicon layer 4 from beingoxidized and to be difficult to remove by the subsequent step of wetetching, for example, 4000 to 5000 angstroms in thickness.

<D-2. Characteristic Function and Effect>

As above described, the oxide film 43 is formed extending from the edgesection to the lower major surface of the silicon substrate 300, and hassuch a thickness that it is not easily removed by wet etching. Thus, ifthe oxidizing agent enters the gaps between the single crystal grains ofthe polysilicon layer 4 to form the oxide film surrounding the singlecrystal grains, the polysilicon layer 4 is prevented from being liftedoff into particles during wet etching.

<E. Fifth Preferred Embodiment>

<E-1. Processing Method>

A fifth preferred embodiment of the semiconductor substrate and themethod of fabricating the semiconductor device according to the presentinvention is described with reference to FIGS. 21 through 23 which showthe processing steps.

First, as shown in FIG. 21, an oxide film 51 is formed so as to entirelycover the SOI substrate 10. The oxide film 51 may be formed by thermallyoxidizing the SOI substrate 10 at a temperature on the order of 700 to1100° C. or by the CVD process at a temperature on the order of 600 to850° C. The polysilicon layer 4 is formed on the edge section and lowermajor surface of the silicon substrate 1 to constitute the PBCstructure.

As depicted in FIG. 22, a resist mask R5 is selectively formed so as tocover the edge section and lower major surface of the SOI substrate 10.In the central section of the upper major surface of the SOI substrate10, the resist mask R5 is not formed and the oxide film 51 is exposed.The exposed oxide film 51 is selectively removed by wet etching.

Thereafter, the resist mask R5 is removed to provide a structure asshown in FIG. 23 wherein the oxide film 51 covers the edge section andlower major surface of the silicon substrate 1 and the SOI layer 3 isexposed in the central section of the upper major surface of the SOIsubstrate 10.

<E-2. Characteristic Function and Effect>

The step of suitably reducing the thickness of the exposed SOI layer 3in accordance with the specs of a desired semiconductor device issimilar to that of the first preferred embodiment described withreference to FIGS. 5 and 6, and the description thereof will bedispensed with herein. The oxide film 51 is not completely removed inthe step of reducing the thickness of the SOI layer 3. The fifthpreferred embodiment does not present the problem that the SOI layer 3under the oxide film 51 is lifted off into particles suspended in theetchant, to prevent the formation failures of the semiconductor elementsresulting from the presence of the particles, thereby increasing thefabrication yield.

Moreover, setting the thickness of the oxide film 51 to a thickness forthinning the SOI layer 3 eliminates the need to form the oxide filmagain for the thinning step. To this end, the thickness of the oxidefilm 51 should be set so that the thickness of the SOI layer 3 conformsto the specs of the desired semiconductor device.

<F. Sixth Preferred Embodiment>

<F-1. Processing Method>

A sixth preferred embodiment of the semiconductor substrate and themethod of fabricating the semiconductor device according to the presentinvention is described with reference to FIGS. 24 through 28 which showthe processing steps.

First, as shown in FIG. 24, an oxide film 61 (first oxide film) having athickness of, for example, 1600 angstroms is formed so as to entirelycover the SOI substrate 10. The thickness of the oxide film 61 is set tothe thickness for thinning the SOI layer 3. That is, the thickness ofthe oxide film 61 is set so that the thickness of the SOI layer 3conforms to the specs of a desired semiconductor device. FIG. 25 showsthe details of an area Z shown in FIG. 24.

The oxide film 61 may be formed by thermally oxidizing the SOI substrate10 at a temperature on the order of 700 to 1100° C. or by the CVDprocess at a temperature on the order of 600 to 850° C. The polysiliconlayer 4 is formed on the edge section and lower major surface of thesilicon substrate 1 to constitute the PBC structure.

Then, as shown in FIG. 26, a nitride film 62 (oxidation-resistant film)having a thickness of 1000 to 4000 angstroms is formed so as to coverthe central section of the upper major surface (on which the activeregions are to be formed) of the SOI substrate 10. The process forforming the nitride film 62 comprises forming the nitride film 62 so asto entirely cover the SOI substrate 10 by the CVD process, forming aresist mask so as to cover the central section of the upper majorsurface of the SOI substrate 10, and selectively removing the nitridefilm 62 by dry etching using the resist mask as an etching mask.

In the step shown in FIG. 27, an oxide film 63 (second oxide film) isformed by oxidizing the edge section and lower major surface of the SOIsubstrate 10. This oxidizing step is performed in a manner similar tothe LOCOS oxidation by using the oxide film 61 exposed on the edgesection and lower major surface of the SOI substrate 10 as an underlyingoxide film. The conditions of this oxidizing step to be selected aresuch that all of the SOI layer 3 except under the nitride film 62 isoxidized. For example, when the SOI layer 3 under the nitride film 62has a thickness of 2000 angstroms, the oxide film 63 should have athickness of not less than 5000 angstroms.

As depicted in FIG. 28, the nitride film 62 is removed, and thereafterthe oxide film 61 which has been positioned under the nitride film 62 isremoved. This provides the SOI layer 3 having the thickness whichconforms to the specs of the desired semiconductor device.

As above discussed, setting the thickness of the underlying oxide filmto the thickness suitable for thinning the SOI layer may reduce thenumber of times the oxide film is formed. It is needless to say thatthis process may be applied to the second, third and fifth preferredembodiments of the present invention described above.

<F-2. Characteristic Function and Effect>

In the step of thinning the SOI layer 3, as above described, thethickness of the oxide film 63 formed on the edge section and lowermajor surface of the SOI substrate 10 is also reduced. However, sincethe thickness of the oxide film 63 is originally greater than that ofthe oxide film 61, the oxide film 63 is not completely removed duringthe etching of the oxide film 61. Further, the oxide film 63 is formedso that the SOI layer 3 is not left on the edge section and lower majorsurface of the SOI substrate 10. This eliminates the drawback that partof the SOI layer 3 is lifted off into particles suspending in theetchant to prevent the formation failures of the semiconductor elementsresulting from the presence of the particles, increasing the fabricationyield.

Additionally, the sixth preferred embodiment requires two steps offorming the oxide film, achieving the reduction in the number of steps.The sixth preferred embodiment requires only one step of oxidationassociated with the thinning of the SOI layer, providing bettercontrollability of the thickness of the SOI layer.

<G. Seventh Preferred Embodiment>

In the above described first to sixth preferred embodiments, theprocessing of the edge section of the SOI substrate or the bulk siliconsubstrate is solely performed. It is, however, needless to say that thesteps of fabricating semiconductor elements in the central section ofthe upper major surface (on which the active regions are to be formed)of the SOI substrate or the bulk silicon substrate may be performed atthe same time that the steps of processing the edge section areperformed.

A process for performing the steps of fabricating semiconductor elementsin the central section at the same time that the steps of processing theSOI substrate of the sixth preferred embodiment are performed will bedescribed with reference to FIGS. 29 through 32. A process forperforming the steps of fabricating semiconductor elements in thecentral section at the same time that the steps of processing the SOIsubstrate of a combination of the fifth and sixth preferred embodimentsare performed will be described with reference to FIGS. 33 through 39.

Like reference characters are used to designate parts identical withthose of the fifth and sixth preferred embodiments, and duplicatedescription will be dispensed with.

<G-1. Modification of Sixth Preferred Embodiment>

First, as shown in FIG. 29, the oxide film 61 having a thickness of, forexample, 1600 angstroms is formed so as to entirely cover the SOIsubstrate 10. The polysilicon layer 4 is formed on the edge section andlower major surface of the silicon substrate 1 to constitute the PBCstructure. The nitride film 62 having a thickness of 1000 to 4000angstroms is formed so as to entirely cover the oxide film 61.

A resist mask R6 is selectively formed on the central section of theupper major surface of the SOI substrate 10.

Then, dry etching is performed using the resist mask R6 as an etchingmask to selectively remove the nitride film 62 so that the nitride film62 is left only under the resist mask R6.

In the step shown in FIG. 30, the oxide film 63 is formed by oxidizingthe edge section and lower major surface of the SOI substrate 10. Thisoxidizing step is performed in a manner similar to the LOCOS oxidationby using the oxide film 61 exposed on the upper major surface centralsection, edge section and lower major surface of the SOI substrate 10 asan underlying oxide film. The conditions of this oxidizing step to beselected are such that all of the SOI layer 3 except under the nitridefilm 62 is oxidized. For example, when the SOI layer 3 under the nitridefilm 62 has a thickness of 2000 angstroms, the oxide film 63 should havea thickness of not less than 5000 angstroms. Part of the oxide film 63which is positioned on the central section of the upper major surface ofthe SOI substrate 10 serves as a field oxide film (LOCOS oxide film).

In the step shown in FIG. 31, the nitride film 62 is removed, andthereafter the oxide film 61 which has been positioned under the nitridefilm 62 is removed. This provides the SOI layer 3 having the thicknesswhich conforms to the specs of the desired semiconductor device. At thistime, the thickness of the oxide film 63 formed on the edge section andlower major surface of the SOI substrate 10 is also reduced. However,since the thickness of the oxide film 63 is originally greater than thatof the oxide film 61, the oxide film 63 is not completely removed duringthe etching of the oxide film 61.

FIG. 32 is a plan view of the SOI substrate 10 as viewed from above theupper major surface. The oxide film 63 is formed on the edge section ofthe SOI substrate 10, and active regions AR are formed in the centralsection thereof as shown in FIG. 32.

Subsequently, semiconductor elements are fabricated into the respectiveactive regions AR defined by the field oxide film. During thefabrication, the oxide film 63 covers the edge section and lower majorsurface of the SOI substrate 10, and the oxide film 63 is formed so thatthe SOI layer 3 is not left in the edge section and lower major surfaceof the SOI substrate 10. This eliminates the drawback that part of theSOI layer 3 is lifted off into particles suspending in the etchant toprevent the formation failures of the semiconductor elements resultingfrom the presence of the particles, increasing the fabrication yield.

<G-2. Modification of Combination of Fifth and Sixth PreferredEmbodiments>

First, as shown in FIG. 33, the oxide film 61 having a thickness of, forexample, 1600 angstroms is formed so as to entirely cover the SOIsubstrate 10. The polysilicon layer 4 is formed on the edge section andlower major surface of the silicon substrate 1 to constitute the PBCstructure. The nitride film 62 having a thickness of 1000 to 4000angstroms is formed so as to entirely cover the oxide film 61.

A resist mask R7 is selectively formed so as to cover the edge sectionand lower major surface of the nitride film 62 as shown in FIG. 34. Inthe central section of the upper major surface of the SOI substrate 10,the resist mask R7 is not formed and the nitride film 62 is exposed. Theexposed nitride film 62 is removed by dry etching, and the oxide film 61thereunder is removed by wet etching. Then, the SOI layer 3 is exposed.

As shown in FIG. 35, an oxide film 71 having a thickness of, forexample, 300 angstroms is formed on the central section of the uppermajor surface. The oxide film 71 may be formed by thermally oxidizingthe SOI substrate 10 at a temperature on the order of 700 to 1100° C. orby the CVD process at a temperature on the order of 600 to 850° C.Subsequently, a nitride film 72 (oxidation-resistant film) having athickness of, for example, 1500 angstroms is formed so as to entirelycover the SOI substrate 10. A resist mask R8 is selectively formed onthe central section of the upper major surface of the SOI substrate 10.

Dry etching is performed using the resist mask R8 as an etching mask toselectively remove the nitride film 72 so that the nitride film 72 isleft only under the resist mask R8 as shown in FIG. 36. In the edgesection of the SOI substrate 10, the nitride film 72 is removed, but thenitride film 62 which has been positioned under the nitride film 72 isleft.

In the step shown in FIG. 37, an oxide film 73 is formed by oxidizingthe edge section and lower major surface of the SOI substrate 10. Thisoxidizing step is performed in a manner similar to the LOCOS oxidationby using the oxide film 71 exposed in the upper major surface centralsection, edge section and lower major surface of the SOI substrate 10 asan underlying oxide film. The conditions of this oxidizing step to beselected are such that all of the SOI layer 3 except under the nitridefilm 72 is oxidized. For example, when the SOI layer 3 under the nitridefilm 72 has a thickness of 2000 angstroms, the oxide film 63 should havea thickness of not less than 5000 angstroms. Part of the oxide film 73which is positioned on the central section of the upper major surface ofthe SOI substrate 10 serves as a field oxide film (LOCOS oxide film).Part of the oxide film 73 which is positioned on the edge section isjoined to the oxide film 61.

In the step shown in FIG. 38, the nitride film 72 is removed by dryetching. At this time, the nitride film 62 is also etched on the edgesection of the SOI substrate 10. However, since the thickness of thenitride film 62 is greater than that of the nitride film 72, the nitridefilm 62 is not completely removed.

FIG. 39 shows MOS transistors formed in the active regions of the SOIsubstrate 10. With reference to FIG. 39, after the MOS transistors MTare formed in the active regions, the upper major surface of the SOIsubstrate 10 is covered with an interlayer insulating film IL, and aresist mask R9 is selectively formed on the interlayer insulating filmIL. The interlayer insulating film IL is selectively removed using theresist mask R9 as an etching mask. Then, the nitride film 62 whichcovers the edge section and lower major surface eliminates the drawbackthat part of the SOI layer 3 is lifted off into particles suspending inthe etchant.

In the above description, a double-layer structure comprised of theoxide film and the nitride film is used in the edge section of the SOIsubstrate 10. However, a triple-layer structure comprised of an oxidefilm, a nitride film, and an oxide film may be used in place of thedouble-layer structure. The use of the triple-layer structure allows thetop oxide film to act as a mask against the etching of the nitride filmduring the removal of the nitride film after the LOCOS oxidation,preventing the nitride film from being etched in the edge section.

Although the nitride film is formed in position to prevent oxidation inthe first to seventh preferred embodiments of the present invention asabove described, the film functioning as a mask against oxidation is notlimited to the nitride film. Any oxidation-resistant film may be usedwhich is impermeable to oxygen serving as an oxidizing agent and whichitself is not oxidized.

While the invention has been described in detail, the foregoingdescription is in all aspects illustrative and not restrictive. It isunderstood that numerous other modifications and variations can bedevised without departing from the scope of the invention.

We claim:
 1. A method of fabricating a semiconductor device using asemiconductor substrate having a first major surface, a second majorsurface opposite from said first major surface, and a side surface, saidfirst major surface including a central section in which active regionsare to be formed and a peripheral section, said peripheral section andsaid side surface defining an edge section, said substrate comprising anSOI layer formed on an insulating layer, said method comprising thesteps of: (a) forming a first oxide film so as to cover said centralsection and said edge section of said semiconductor substrate; (b)forming an oxidation-resistant film entirely on said first oxide film insaid central section; (c) further oxidizing said edge section of saidsemiconductor substrate using said oxidation-resistant film as a mask toform a second oxide film in said edge section, said second oxide filmbeing thicker than said first oxide film; and (d) forming semiconductorelements in said active regions.
 2. The method according to claim 1,wherein said semiconductor substrate is an SOI substrate formed by anSIMOX technique; wherein said semiconductor substrate comprises a buriedoxide film and an SOI layer formed in a sequentially stacked relation inthe entire first major surface; and wherein said step (c) comprises thestep of (c-1) forming said second oxide film so as to completely oxidizesaid SOI layer extending in said edge section and to oxidize part ofsaid edge section which has not been oxidized.
 3. The method accordingto claim 1, wherein said semiconductor substrate is an SOI substrateformed by a bonding technique; wherein said semiconductor substratecomprises an on-substrate oxide film and an SOI layer formed in asequentially stacked relation on the entire first major surface; andwherein said step (c) comprises the step of (c-1) forming said secondoxide film so as to completely oxidize said SOI layer extending in saidedge section and to oxidize part of said edge section which has not beenoxidized.
 4. The method according to claim 3, wherein said step (a)comprises the step of forming said first oxide film so that thethickness of said SOI layer in said central section is reduced to athickness conforming to formation of semiconductor elements.
 5. Themethod according to claim 4, wherein said step (b) comprises the step offorming a pattern of said oxidation-resistant film in accordance withthe pattern for forming a field oxide film defining said active regionsin said central section, and wherein said step (c) comprises the step offorming said second oxide film as said field oxide film in accordancewith said pattern of said oxidation-resistant film in said centralsection.
 6. The method according to claim 1, wherein said semiconductorsubstrate is a bulk silicon substrate; wherein said semiconductorsubstrate comprises a polysilicon layer formed on said edge section andsaid second major surface; and wherein said step (c) comprises the stepof (c-1) forming said second oxide film so that said polysilicon layeris not completely oxidized.
 7. A method of fabricating a semiconductordevice using a semiconductor substrate comprising an SOI layer andhaving a first major surface, a second major surface opposite from saidfirst major surface, and a side surface, said first major surfaceincluding a central section in which active regions are to be formed anda peripheral section, said peripheral section and said side surfacedefining an edge section, said method comprising the steps of: (a)forming a first oxide film so as to cover said central section and saidedge section of said semiconductor substrate and reducing the thicknessof said SOI layer to a thickness conforming to formation ofsemiconductor elements; (b) selectively forming an oxidation-resistantfilm on said first oxide film in said central section; (c) furtheroxidizing said edge section of said semiconductor substrate using saidoxidation-resistant film as a mask to form a second oxide film in saidedge section, said second oxide film being thicker than said first oxidefilm; and (d) forming semiconductor elements in said active regions. 8.The method according to claim 7, wherein said step (b) comprises thestep of forming a pattern of said oxidation-resistant film in accordancewith the pattern for forming a field oxide film defining said activeregions in said central section, and wherein said step (c) comprises thestep of forming said second oxide film as said field oxide film inaccordance with said pattern of said oxidation-resistant film in saidcentral section.
 9. A method of fabricating a semiconductor device usinga semiconductor substrate having a first major surface, a second majorsurface opposite from said first major surface, and a side surface, saidfirst major surface including a central section in which active regionsare to be formed and a peripheral section, said peripheral section andsaid side surface defining an edge section, said method comprising thesteps of: (a) forming an oxide film so as to cover said central sectionand said edge section of said semiconductor substrate; (b) forming aresist mask on said oxide film except in said central section; (c)selectively removing said oxide film in said central section using saidresist mask as an etching mask to expose said semiconductor substrate,with said oxide film left in said edge section; and (d) formingsemiconductor elements in said active regions.
 10. The method accordingto claim 9, further comprising the step of (e) forming anoxidation-resistant film on said oxide film in said edge section. 11.The method according to claim 10, wherein said semiconductor substrateis an SOI substrate formed by an SIMOX technique; wherein saidsemiconductor substrate comprises a buried oxide film and an SOI layerformed in a sequentially stacked relation in the entire first majorsurface; and wherein said step (a) comprises the step of forming saidoxide film so that the thickness of said SOI layer in said centralsection is reduced to a thickness conforming to formation ofsemiconductor elements.
 12. The method of fabricating a semiconductordevice using a semiconductor substrate having a first major surface, asecond major surface opposite from said first major surface, and a sidesurface, said first major surface including a central section in whichactive regions are to be formed and a peripheral section, saidperipheral section and said side surface defining an edge section, andsemiconductor substrate being an SOI substrate formed by a SIMOXtechnique, said semiconductor substrate including a buried oxide filmand an SOI layer formed in a sequentially stacked relation in the entirefirst major surface, said method comprising the steps of: (a) forming afirst oxide film so as to cover said central section and said edgesection of said semiconductor substrate; (b) forming a resist maskentirely on said first oxide film in said central section; (c)selectively removing said first oxide film and said SOI layer in saidedge section of said semiconductor substrate using said resist mask asan etching mask to expose said buried oxide film; (d) further oxidizingsaid first oxide film under said resist mask to form a second oxide filmthicker than said first oxide film and to increase the thickness of saidburied oxide film exposed; and (e) forming semiconductor elements insaid active regions.
 13. The method according to claim 12, wherein saidstep (d) comprises the step of forming said second oxide film so thatthe thickness of said SOI layer in said central section is reduced to athickness conforming to formation of semiconductor elements.
 14. Amethod of fabricating a semiconductor device using a semiconductorsubstrate having a first major surface, a second major surface oppositefrom said first major surface, and a side surface, said first majorsurface including a central section in which active regions are to beformed and a peripheral section, said peripheral section and said sidesurface defining an edge section, said semiconductor substrate being anSOI substrate formed by a SIMOX technique, said semiconductor substrateincluding a buried oxide film and an SOI layer formed in a sequentiallystacked relation in the entire first major surface, said methodcomprising the steps of: (a) forming a first oxide film so as to coversaid central section and said edge section of said semiconductorsubstrate; (b) selectively forming a resist mask on said first oxidefilm in said central section; (c) selectively removing said first oxidefilm, said SOI layer and said buried oxide film in said edge section ofsaid semiconductor substrate by dry etching using said resist mask as anetching mask to expose an underlying substrate under said SOI layer; (d)further oxidizing said first oxide film under said resist mask to form asecond oxide film thicker than said first oxide film and to form a thirdoxide film on said underlying substrate exposed; and (e) formingsemiconductor elements in said active regions.
 15. The method accordingto claim 14, wherein said step (d) comprises the step of forming saidsecond oxide film so that the thickness of said SOI layer in saidcentral section is reduced to a thickness conforming to formation ofsemiconductor elements.